Various circuits, including many power supply and motor control circuits, utilize transistor bridge arrangements to selectively couple a load to a power supply. In general, one terminal of the power supply is designated as the circuit common or ground, and transistors that couple a load to the circuit ground are referred to as low-side switches, whereas transistors that couple a load to the other terminal of the power supply or an intermediate potential are referred to as high-side switches. Since the emitter or source of a low-side transistor is referenced to circuit ground, its gate drive circuit may also be referenced to ground, and a simple and inexpensive circuit design may be used. However, the emitter or source of a high-side transistor floats with respect to ground, and its gate drive circuit must therefore be isolated from ground. One technique commonly employed with insulated gate transistor bridges utilizes a capacitor and diode to create a floating power supply for the gate drive of the high-side transistor. This technique, illustrated in FIG. 1, is referred to as bootstrapping, and eliminates the need for expensive transformer isolated power supplies. Referring to FIG. 1, a Low Level Controller 10 generates control signals L1 and H1 for biasing low-side and high-side bridge transistors Q1 and Q2 alternately on and off to alternately couple an electrical load LOAD1 to a ground potential defined by the negative terminal of battery 12 and a source potential VS defined by the positive terminal of battery 12. The Low Level Controller 10 is powered by Logic Level Power Supply 14, which develops a logic level voltage Va, such as 5 VDC. The control signal L1 is applied to gate drive amplifier U1, which switches the gate of low-side transistor Q1 between the logic level voltage Va and ground potential, according to the logic state of the control signal L1. Similarly, the control signal H1 controls the operation of gate drive amplifier U2 for high-side transistor Q2, but in this case a level shifter (LS) 16 is needed since the source of high-side transistor Q2 floats at load potential VL1. A by-pass capacitor C1 supplies a gate drive potential Vb relative to the source of high-side transistor Q2, and Logic Level Power Supply 14 charges capacitor C1 substantially to voltage Va through diode D1 during on periods of low-side transistor Q1. If desired, a by-pass capacitor C2 may also be used to establish a gate drive potential for gate drive amplifier U1, as shown. The drive circuit for high-side transistor Q2 is said to be bootstrapped because its gate drive potential Vb shifts upward with the load potential VL1 when low-side transistor Q1 transitions from on to off. At such point, the gate drive potential Vb is higher than the logic level voltage Va, and the diode D1 is reverse biased. When control signal H1 changes states to turn on high side transistor Q2, the by-pass capacitor C1 supplies the required gate drive energy.
While conventional bootstrapping can be advantageously used in single power supply bridge circuits such as shown in FIG. 1, it can only be used in multiple power supply bridge circuits by providing separate logic level voltages for each power supply, which of course, entails significant expense. Accordingly, what is needed is a simple and inexpensive gate driver circuit having multiple outputs for driving bridge transistors of multiple power supply bridge circuits.